Method and circuit arrangement for driving a laser diode

ABSTRACT

The invention relates to a process and circuitry for controlling a laser diode. To compensate for the temperature-dependent drop in the light intensity produced by the laser diode, the laser diode is controlled by a voltage generator with an adjustable internal resistance. A rise in the driver current of the laser diode as the temperature rises is set by the adjustable internal resistance in such a way that the rise in the light intensity produced determined by the rise in the driver current compensates for the temperature-dependent fall in the light intensity.

BACKGROUND OF THE INVENTION

The invention refers to the fields of technology herein laser diodes areutilized, for example to reproduction technology, and is directed to amethod and to a circuit arrangement for driving a laser diode in arecording device such as a laser printer or laser recorder.

In such a recording device, the light output by the laser diode andmodulated with the information to be recorded is shaped into a recordingbeam with optical means and is deflected point-by-point and line-bylineacross a recording medium with a deflection system during recording.

In most cases, a constant current source is employed for driving thelaser diode. The constant current source generates a constant drivercurrent that is modulated by an image signal that contains theinformation to be recorded and is supplied to the laser diode.

The laser diode works in switched mode fashion for recording lineinformation, wherein the driver current modulated by a two-level imagesignal switches the laser diode on and off.

In order to achieve a high recording quality in the recording of lineinformation, the laser diode must switch quickly and the light leveloutput by the laser diode must be constant in the on-time intervals inorder to achieve a uniform illumination.

Based on its very nature, the laser diode does not satisfy the demandfor a constant light level in the on-time intervals.

The output light power is temperature dependent, namely such that thelight power decreases with rising operating temperature of the laserdiode.

The gradual rise in the operating temperature in the substrate of thelaser diode and the drop of the output light power caused as a resultthereof can in fact be compensated by regulating the housingtemperature; in a modulation or, switched mode, however, the laser diodestill has a dynamic temperature effect. The cause of the dynamictemperature affect is the temperature change of the laser transition inthe chip dependent on the modulation or, on the image signal thatcontrols the modulation. Due to this temperature dependency of the lasertransition, a temperature difference between substrate and lasertransition continues to exist even when the housing temperature isregulated and an internal temperature compensation process is theconsequence. Given a laser diode working in switched mode, this innertemperature compensation process leads to a variation of the outputlight power dependent on the image signal in such a way that the outputlight power rises above the nominal level in the respective turn-on timeand then only gradually reaches the nominal level within the individualon-time intervals. Due to this effect, a disturbing lag effect that hasa considerable influence on the recording quality arises on therecording medium when recording line information.

Various measures for correcting the light power output of a laser diodeare already known.

For example, it has already been proposed to measure the light poweroutput by the laser diode within the respective time interval requiredfor the recording of a line and to control the light power via thedriver current dependent on the measured result.

GB Patent 21 01 851 likewise already discloses that the output lightpower be measured with a photodiode (monitor diode) integrated in thelaser diode within or outside the time interval required for therecording of a line, to calculate correction values from therespectively measured light power, to intermediately store thecorrection values line-byline in sample-and-hold circuits, and tocontrol the light power via the driver current dependent on the storedcorrection values.

A regulation for correcting the light power output has the disadvantagethat it is involved and that stability problems arise due to the controlloops, and the switching speed of the laser diodes is reduced.

It is therefore an object of the invention to specify a method and acircuit arrangement for driving a laser diode with which a simplecorrection of the output light power and an improved switching behaviorare achieved.

According to the method of the invention for driving a laser diode, thelaser diode is charged by a driver current which defines an output lightpower, the output light power being dependent upon and dropping with arise of internal temperature of the laser diode. For compensating thetemperature-dependent drop in light power, the laser diode is driven bya voltage generator having a variable internal resistance R_(i). Theinternal resistance is calculated according to the equation: ##EQU1##wherein σ is the efficiency of the laser diode, T_(KU) is a temperaturecoefficient based on an on-state voltage of the laser diode, T_(kp) is atemperature coefficient based on a power of the laser diode, and R_(Di)is an internal resistance of the laser diode, a rise of the drivercurrent given rising temperature of the laser diode being limited by theinternal resistance R_(i) such that a rise in the output light powercaused by a rise of the driver current compensates thetemperature-dependent drop in light power.

As a result of these techniques, it is particularly a correction of thedisturbing lag effect that can be achieved given employment of the laserdiodes in recording devices without deteriorating the switching speedand a good recording quality given high recording speed can be achievedoverall.

The invention shall be set forth in greater detail below with referenceto FIGS. 1 through 3.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fundamental exemplary embodiment of a circuit arrange alaser diode;

FIGS. 2a to 2 f are a time diagram; and

FIGS. 3a-3b are exemplary embodiments of voltage generators havingpositive and negative internal resistance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a fundamental exemplary embodiment a circuit arrangementfor driving a laser diode. The circuit arrangement 1 for driving a laserdiode 2 is essentially composed of a voltage source 3 having a variableinternal resistance R,. In switched mode, the voltage source 3 iscontrolled by a two-level image signal B which modulates the generatorvoltage U_(O) of the voltage source 3 dependent on the information to berecorded. During continuous mode operation, the voltage source 3 outputsa constant generator voltage U_(O).

The real laser diode 2 is represented in the equivalent circuit diagramas a series circuit composed of an ideal laser diode 2' and of theinternal resistance R_(Di) of the real laser diode. The generatorvoltage U_(O) generates a driver current I_(T) for the laser diode 2,this driver current I_(T) being dependent on the generator voltageU_(O), on the on-state voltage U_(D) of the laser diode 2' and theinternal resistances R₁ and R₀₁ according to equation (1). ##EQU2##

The light power P output by the laser diode 2 is proportional to thedriver current I_(T). A variation of the driver current ΔT_(T) leads toa variation of the output light power ΔP according to equation 2.##EQU3## In equation (2), "σ" is the efficiency of the laser diode 2that, for example, amounts to 0.3 through 1 mW/mA.

Due to the temperature behavior of the laser diode 2 set forth in theintroduction to the specification, the output light power P and theon-state voltage U_(D) of the laser diode 2 drop with rising temperatureT.

A temperature change ΔT leads to a modification of the output lightpower ΔP_(U) according to equation (3). ##EQU4## In equation (3), T_(KP)=dP/dT is the temperature coefficient referred to the power which andfor example, can be -0.4 through -0.7 mw/°C.

A temperature-dependent change in the on-state voltage U_(D) Of thelaser diode 2 leads to a rise in the driver current I_(T) due to theinventive drive of the laser diode 1 with a voltage source 3 accordingto equation (1). By varying the on-state voltage ΔU_(D) given atemperature change ΔT, a variation of the driver current ΔT_(t) derivesaccording to equation (4). ##EQU5## In equation (5), T_(KU) =dU_(D) /dTis the temperature coefficient referred to the voltage which, forexample, is -1.7 through -1.3 mW/°C.

Due to the type of drive for the laser diode, the rise in the drivercurrent IT due to the temperature-dependent decrease in the on-statevoltage UD can be set such according to the idea of the invention withthe variable internal resistance R₁ such that the temperature-dependentdrop of the light power P is corrected, whereby a constant nominal levelis advantageously achieved in continuous mode operation and in theon-time intervals of switched mode.

A compensation of the temperature-dependent drop in light power isachieved when ΔP₁ =-ΔP_(U) applies. With equation (4), the internalresistance R₁ required for the compensation can be calculated therefromaccording to equation (5) as follows: ##EQU6##

In practice, the calculated internal resistance R, can be positive ornegative, so that the drive of the laser diode 2 must occur with avoltage source having positive or having negative internal resistance.Exemplary embodiments of a voltage source having positive and negativeinternal resistance are shown and described in FIG. 3.

In diagrams FIGS. 2a), 2b), and 2c) for the switched mode, thechronological curves of the image signal B (diagram 2a), of the drivercurrent I_(T) (diagram 26 and the light power P output by the laserdiode 2 (diagram 2c) are shown on the basis of the temperaturecompensation process at the laser transition set forth in theintroduction to the specification given traditional drive of the laserdiode. Diagram 2c) shows that the output light power P respectivelyrises above the nominal power level (100%) at the turn-on time and onlygradually reaches the nominal power level within the respective on-timeintervals, as a result whereof the disturbing lag effect alreadylikewise mentioned in the introduction to the specification arises onthe recording medium.

The result achieved by the inventive drive of the laser diode 2 is shownin diagram 2e) and 2f) .

Diagram 2d shows the driver current I*_(T) that occurs 2f voltage driveof the laser diode and diagram 2f shows the chronological curve of thecorrected light power P*.

FIG. 3a shows an exemplary embodiment of a voltage source having avariable, positive internal resistance R_(i) that is composed of a wiredoperational amplifier 4.

FIG. 3b shows an exemplary embodiment of a voltage source having avariable, negative internal resistance R_(i) that differs from theexemplary embodiment shown in FIG. 3a in that the voltage source shownin FIG. 3b is followed by a circuit 5 that simulates a negative internalresistance.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that I wish to includewithin the claims of the patent warranted hereon all such changes andmodifications as reasonably come within my contribution to the art.

I claim:
 1. A method for driving a laser diode, comprising the stepsof:driving the laser diode with a driver current that defines an outputlight power, the output light power dropping in dependence upon a riseof internal temperature of the laser diode; compensating thetemperature-dependent drop in light power by driving the laser diodewith a voltage generator having a variable internal resistance R_(i), arise of the driver current given rising internal temperature of thelaser diode being limited by the internal resistance R_(i) such that arise in the output light power caused by a rise of the driver currentcompensates said temperature-dependent drop in light power; andcalculating the internal resistance R_(i) according to the equation:##EQU7## wherein ρ is an efficiency of the laser diode, T_(Ku) is atemperature coefficient with reference to an on-state voltage of thelaser diode, T_(Kp) is a temperature coefficient relating to a power ofthe laser diode, and R_(Di) is an internal resistance of the laserdiode.
 2. A method for driving a laser diode, comprising the stepsof:driving the laser diode with a driver current that defines an outputlight power, the output light power dropping and being dependent upon arise of temperature of the laser diode; compensating thetemperature-dependent drop in light power by driving the laser diodewith a voltage generator having an internal resistance R_(i) ; andcalculating the internal resistance R_(i) according to the equation##EQU8## wherein ρ is an efficiency of the laser diode, T_(Ku) is atemperature coefficient with reference to an on-state voltage of thelaser diode, T_(Kp) is a temperature coefficient relating to a power ofthe laser diode, and R_(Di) is an internal resistance 2f the laserdiode.
 3. A circuit arrangement for driving a laser diode,comprising:generator means for generating a driver current for definingan output light power of the laser diode, the output light power of thelaser diode being dependent upon and dropping with a rise in internaltemperature thereof; means for compensation of the temperature-dependentdrop of the light power, said means comprising a voltage generatorhaving a voltage source and a variable internal resistance Ri; and theinternal resistance Ri of said voltage generator being calculatedaccording to an equation: ##EQU9## where ρ is an efficiency of the laserdiode, T_(Ku) is a temperature coefficient based on an on-state voltageof the laser diode, T_(KE) is a temperature coefficient based on a powerof the laser diode, and R_(Di) is an internal resistance of the laserdiode.
 4. A circuit arrangement for driving a laser diode,comprising:generator means for generating a driver current for definingan output light power of the laser diode, the output light power of thelaser diode being dependent upon and dropping with a rise of temperaturethereof; means for compensation of the temperature-dependent drop of thelight power, said means comprising a voltage generator having a voltagesource and an internal resistance R_(i), and the internal resistanceR_(i) of said voltage generator being calculated according to anequation: ##EQU10## where Υ is an efficiency of the laser diode, T_(KU)is a temperature coefficient based on an on-state voltage of the laserdiode, T_(Kp) is a temperature coefficient based on a power of the laserdiode, and R_(DI) is an internal resistance of the laser diode.